Please see the red arrow beneath. How can 'social meaning of an action' be the Formal Cause of the human action, when it doesn't relate to the form, shape or structure of the human action?
If we go back to what Aristotle actually said (Physics II.3, Metaphysics, H.4) the material cause is the matter or substance or stuff of which a thing is made; the formal cause is the form of the thing; the efficient cause is what brought it into existence; and the final cause is its purpose, the reason why it was brought into existence. So a statue is made of stone (material cause), it is a statue of Hermes by virtue of its structure or organisation (formal cause), it is brought into existence as a statue of Hermes by the activities of the sculptor (efficient cause); and the purpose is the sculptor's intention to bring into existence a statue of Hermes, the sculptor's intention to realise the blueprint in her/ his mind (final cause).
This will serve as a rough account. You will be able to see that the formal cause in this account has to undergo drastic transformation if it is to identify with Capaldi's 'social meaning of an action'. Capaldi is stretching Aristotle beyond anything Aristotle would recognise as a formal cause.
To be constructive, however, what kind of case can be made ? One could say that the formal cause gives the 'essence', the defining feature, of the thing. The lump of stone can be classified by virtue of its form (structure) as a statue of Hermes. This is what it essentially is. In a kind of loose parallel, the social meaning of an action is the essence of the action, its defining feature. By virtue of its social meaning it is an act of greeting, say, rather than of insult or indifference.
I would not put Aristotle's language to Capaldi's use but if he chooses to appropriate Aristotle in this way, doubtless he has his reasons.
Note that Capaldi and Smit are discussing "human actions" and not statues or acorns. We should still be able to come up with explanations, or causes, for these as well.
Andrea Falcon describes a formal cause as “the account of what-it-is-to-be”.
A formal causes tells us what the human action is. This "what" could be described as the "social meaning" of that action.
Capaldi, N., Smit, M. (2007). The art of deception: an introduction to critical thinking. Prometheus Books.
Falcon, Andrea, "Aristotle on Causality", The Stanford Encyclopedia of Philosophy (Spring 2015 Edition), Edward N. Zalta (ed.), URL = https://plato.stanford.edu/archives/spr2015/entries/aristotle-causality/.